You can't really control your feelings. If there's no spark, there isn't really anything you can do about it. You are obviously still confused about your ex and might’ve gone into a new relationship prematurely. Maybe the best thing to do is be by yourself for a while, clear your mind and really think about what is best for you. If you aren't committed to your new relationship, then it really isn't fair to lead him on. Although it's unintentional, if you are still thinking about someone else and aren't emotionally invested in this relationship, the best thing to do is to end it before his feelings for you become more intense.